This change makes it so that leaf_walk_rcu takes a tnode and a key instead
of the trie and a leaf.

The main idea behind this is to avoid using the leaf parent pointer as that
can have additional overhead in the future as I am trying to reduce the
size of a leaf down to 16 bytes on 64b systems and 12b on 32b systems.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>